Methylation procedures Hakomori

Methylation of a carbohydrate is most practically achieved by using the Hakomori procedure,9 which has largely replaced the Purdie, Haworth, and Kuhn methylation methods. 

Unlike neutral polysaccharides undermethylated acidic polysaccharides cannot be realkylated directly. The Hakomori procedure leads to esterification of uronic acid residues that on further treatment with base will undergo extensive p-elimination. Therefore the methylated acidic polymer should be carboxyl-reduced with LiAFH4 prior to realkylation. This is illustrated by our work (O Neill et al., 1986a) on the extracellular polysaccharide produced by Alcaligenes (ATCC 31555). After a single methylation an excess of branch points was observed which indicated incomplete methylation of position 2 of (l- 3)-linked glucose residues. To confirm this, the methylated polysaccharide was carboxyl-reduced, ethylated, and the alkylated alditol acetates analyzed by GLC-MS. The... 

An alternative procedure for the permethylation of polysaccharides is the well-known one described by Hakomori, which employs the methyl-sulfinyl carbanion in methyl sulfoxide and methyl iodide. This newer approach was based on the observation that conversion of phenols into phenoxides is greatly enhanced by virtue of the high basicity of this car-... 

The Hakomori method of methylation may be applied to mono- or poly-saccharides, and detailed procedures are available. Methylation in methyl sulfoxide appears to be similar to the method of Kuhn in that furanoside formation at the reducing group is favored, and, where this is undesirable, preliminary methylation with diazomethane is recommended. ... 

NRRL B-742 dextran may be artifactual structures, resulting from incomplete methylation of this highly branched polysaccharide. Dextrans have been methylated most frequently by the Haworth method, " and by procedures that employ sodium and methyl iodide in liquid ammonia. " " Rapid methylation of dextrans has been achieved " through the use of the Hakomori procedure, which utilizes methyl sulfoxide as solvent for the dextran. Careful control of the reaction temperature would, however, appear to be essential when a dextran is methylated in this solvent, as hot methyl sulfoxide has been reported to depolymerize native dextrans." ... 

During the methylation analysis of Combretum nigricans gum by the Hakomori procedure, losses of D-galacturonic acid and L-rhamnose residues were observed, which did not occur in the Haworth or a modified-Haworth procedure.However, during the Haworth procedure, C. nigricans gum was degraded to a greater extent than the gum from A cacia seyal. 

The Hakomori method has also been used extensively in the methylation of carbohydrates however, yields were typically low and non-sugar products were observed. Subsequent studies by Ciucanu and Kerek [40] led to the conclusion that OH and H were more effective bases than the methylsulfinyl carbanion. These authors developed a new method for the permethylation of carbohydrates using CH3I and solid NaOH in DMSO. This reaction is relatively clean, proceeds to completion in about 6-7 min, and gives very high yields ( 98%). The derivatization may also be performed with KOH, NaH (excess) and sodium t-butoxide. Ciucanu and Kerek have extensively reviewed the experimental parameters and have published a table of procedures, reagents, reaction times and yeilds for various other versions of Ihe permethylation reaction [40]. 

---